1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, as well as a developing device and a developer stirring device to be used therein.
2. Description of Related Art
In an image forming apparatus such as a copying machine and a printer, an image is formed through the following processes: an exposing process for forming a desired electrostatic latent image on the surface of a photosensitive drum; a developing process for developing the electrostatic latent image by adhering toner onto the electrostatic latent image; a transferring process for transferring the developed toner image onto desired transfer paper; and a fixing process for fixing the transferred toner image on the paper by heat. The exposing, developing and transferring processes are successively performed by respective devices provided around the photosensitive drum in accordance with the rotation of the photosensitive drum.
Among these devices, a developing device is disposed in the vicinity of the photosensitive drum, and includes a developing roller rotating around an axis parallel to the photosensitive drum, a toner supplier for supplying toner for development to the developing roller and a developer stirring device provided therebetween. The developing roller is a magnetic roller comprising a non-magnetic outer cylinder and a plurality of fixed magnets circumferentially disposed on the inner surface of the outer cylinder. The developer stirring device disposed along the developing roller mixes toner supplied by the toner supplier with a carrier made of magnetic powder to obtain a developer, in which the toner is held on the surfaces of the particles of the carrier.
in such a developing device, the toner and the carrier in the developer are attracted to the outer cylinder of the developing roller by the magnetic force of the fixed magnets, kept adsorbed by the magnetic field circumferentially formed by the fixed magnets, and conveyed in the circumferential direction as the outer cylinder rotates. The toner is thus fed to a position opposing the photosensitive drum, and attracted to the electrostatic latent image formed on the surface of the photosensitive drum by the electrostatic force, thereby separating from the carrier to move onto the photosensitive drum. In this manner, the toner image is formed. The carrier remained on the developing roller is dropped from the outer cylinder during the further rotation and sent back to the developer stirring device for reuse.
In order to attain an excellent development quality in the developing procedure performed in the above-mentioned manner, the developer supplied to each position along the developing roller is required to have a toner concentration as uniform as possible. To meet this demand, a conventionally used developer stirring device includes a pair of stirring paths disposed parallel between the developing roller and the toner supplier. In each of the stirring paths, a stirring screw is provided to convey and stir the developer by the rotation thereof.
The stirring paths are communicated with each other through communicating ports at both the lengthwise ends thereof, and the stirring screws therein are driven to rotate so as to convey the developer in the opposite directions to each other. The toner supplied from the toner supplier to the stirring path along the toner supplier is stirred by the rotation of the stirring screw therein, conveyed while being uniformly mixed with the carrier, and fed to the other stirring path along the developing roller through the communicating ports at one end of the stirring paths. The toner mixed with the carrier, that is, the developer is further stirred and conveyed by the rotation of the stirring screw in the stirring path along the developing roller and goes back to the stirring path along the toner supplier through the communicating ports at the other end of the stirring paths. Then, fresh toner is supplied to the stirring path along the toner supplier, and this cycle is repeated thereafter.
In such a developer stirring device, the developer having a certain toner concentration is fed to the stirring path along the developing roller after being stirred and conveyed through the stirring path along the toner supplier. Such a developer with a uniform toner concentration is distributed over the entire length of throughout the developing roller. Thus, the above-mentioned demand is met, resulting in an excellent development quality.
Larger-scale image forming apparatus, however, are Fabricated these years, and therefore, the length of the developing roller corresponding to the size of the photosensitive drum also becomes larger. When the above described developer stirring device is used in such a large image forming apparatus, the toner concentration in the developer is lowered during the conveyance through the stirring path along the long developing roller. As a result, it is disadvantageously difficult to make uniform the toner concentration at each point throughout the developing roller.
In order to solve this problem, Japanese Patent Application Laid-open Nos. 3-260678 (1991) and 3-274072 (1991) disclose a developer stirring device in which a pair of stirring paths respectively disposed along the toner supplier and the developing roller are communicated with each other not only at the lengthwise ends of the stirring paths but also at the center thereof. In one of the stirring paths, the developer is conveyed from the lengthwise ends toward the center by the rotation of the stirring screw in the stirring path. In the other stirring path, the developer is conveyed from tile center toward the ends thereof.
FIG. 1 shows the operation of such a developer stirring device. As is shown in FIG. 1, the developer stirring device has such a structure that stirring paths 1 and 2 disposed parallel to each other respectively include stirring screws 3 and 4. The stirring paths 1 and 2 are provided between and along a developing roller 5 and a toner supplier 6, and communicated with each other through communicating ports A at both the lengthwise ends and a communicating port B at the center. The stirring screws 3 and 4 have, on the right and left sides of the middle communicating port B, stirring plates 30, 30 and 40, 40, respectively, and the right side plate and the left side plate are spiral in the opposite directions to each other. The stirring screw 3 along the developing roller 5 is driven to rotate around an axis 31 so as to set the conveying direction of the stirring plates 30, 30 toward the center communicating port B, and the stirring screw 4 along the toner supplier 6 is driven to rotate around an axis 41 so as to set the conveying direction of the stirring plates 40, 40 away from the center communicating port B.
In the developer stirring device having the above-mentioned structure, toner is supplied by the toner supplier 6 to the center of the stirring path 2. The supplied toner is stirred and conveyed toward the lengthwise ends by the rotation of the stirring screw 4, during which the toner is mixed with a carrier to obtain a developer. The developer having reached both the ends of the stirring path 2 is fed to the stirring path 1 through the communicating ports A, and then, stirred and conveyed toward the center of the stirring path 1 by the rotation of the stirring screw 3, during which the toner in the developer is attracted by the developing roller 5 to be used in the developing procedure. The developer collected in the middle of the stirring path 1 by the rotation of the stirring screw 3 goes back to the stirring path 2 through the center communicating port B, supplied with fresh toner in the stirring path 2, and conveyed toward both the ends again.
In this manner, the developer in the stirring paths 1 and 2 is circulated as shown with arrows in FIG. 1 through the communicating port B between the stirring paths every time the developer passes through the right or left half of one of the stirring paths. Therefore, the developer is attracted by the developing roller 5 while it is in the left or right half of the stirring path 1. As a result, the loner concentration in the developer little can be stable over the entire length of the developing roller 5, thereby improving the development quality.
Further, the developer stirring device disclosed in Japanese Patent Application Laid-open Nos. 3-260678 (1991) and 3-274072 (1991) has distributing means facing the communicating port B in the stirring path 2, thereby distributing the developer fed from the stirring path 1 through the communicating port B to the right and left portions of the stirring path 2. Thus, the developer passed through the right and left portions is interchanged to prevent difference in the toner concentration in the developer circulating through the right and left portions of the stirring paths.
In addition, in the developer stirring device disclosed in Japanese Laid-open Patent Publication No. 3-260678(1991), an inclined plate is coaxially provided to the stirring screw 4. This inclined plate rotates together with the stirring screw 4, thereby varying the extent of the distribution of the developer to the right and left portions of the stirring path. The developer stirring device disclosed in Japanese Patent Application Laid-open No. 3-274072 (1991) has a regulating member longitudinally reciprocating in accordance with the rotation of the stirring screw 4. Thus, the feeding of the developer to the right and left portions is successively regulated.
Nevertheless, also in the developer stirring device described in the aforementioned publications, after being collected in the middle of the stirring path 1 by the rotation of the stirring screw 3, the developer stops in the vicinity of the communicating port B for a while. When a certain amount of the developer is retained, it is fed to the stirring path 2 at one time. Thus, the developer is irregularly fed to the stirring path 2, resulting in varying the amount of the developer going back to the stirring path 2. Therefore, even when the above-mentioned distributing means works properly, the amount of the developer distributed to the right and left port, ions varies. As a result, the toner concentration in the developer circulating through these portions differs from each other. In a developing device and an image forming apparatus using such a developer stirring device, therefore, it is disadvantageously difficult to attain a desired development quality due to such a difference in the toner concentration.